First check the height of the couplers, they must be level from car to car.
Check the curved "pin" that hangs down below the coupler. If it is too low, it will hit rerailers and parts of switches causing then to uncouple.
Your track must also be level, too abrupt a grade change can cause an uncouple.

Could you describe where your problem occurs.

 Because the Forney is a 2-4-4T, with a rigid wheelbase it will cause the most problems on a tight radius.

I love the Maine 2 foot railroads, so I hate to say this, but you might be better off with one of the 4-4-0s if you need 18 inch radius.
Bachmann did a great job on the Forney. My brass HOn30 Forneys needed as much room as the On30 versions.

 Which caboose?

On the the "side door wood caboose" you would need at least 3.25 inches. That is from the rail top, not including tie/rail height.
The stove stack is the tallest point, so I suppose you could trim it down.

You might want to look at Backwoods Miniatures for small tenders. They have a lot of On30 products for the Bachmann line.

http://www.backwoodsminiatures.com/0n3kits.htm

Heave asked:

Finally but not last, what good is pulpwood?

The prototypes for the pulpwood cars ran on the Maine 2-foot lines. There were many "novelty" mills along the line which used the pulpwood for everday products. In Strong there was a large toothpick mill. Other mills produced shingles, tool handles, spools...and they used the smaller pulpwood.

Jim